1. Field of the Invention
The present invention relates to a roll stand with work rolls which may rest against a back-up roll either directly or through an intermediate roll, particularly work rolls which are axially displaceable in opposite directions.
2. Description of the Related Art
In rolling technology, strips and sheet metal are desired which are planar with respect to their length, width and thickness and which are within increasingly narrow tolerances. While this requirement is solved by known roll stands in a satisfactory manner with respect to the center areas of the strips and sheet metal, section anomalies of the strip occur in the areas near the edges thereof. The reason for this is the drop of the elastic roll deformation from the loaded area to the unloaded area next to the strip. This upper limit beginning in the area of the strip edge leads in connection with local widening of the strip to edge sharpening of the rolled strip and an attendant decrease of the strip thickness in the edge area, i.e., the so-called edge drop.
The better the friction between the rolls and the rolling stock and the thinner the work rolls are, the more sudden this thickness change of the strip will occur. The stronger the rolling stock and the thicker the work rolls are, the greater the thickness drop at the strip edges will be. For example, during cold rolling, this area of roll sharpening of strips of 1250 mm will extend over an edge area having a width of about 15-40 mm which, in the case of poor friction between the rolls and the rolling stock as it frequently occurs as a result of rough rolls in the last stand of a tandem train, can increase to an area more than 40 mm away from the strip edges. Consequently, a portion of this sharpened edge area having an uneven thickness is usually removed by trimming the strips. This trimming of the strip requires another work step and, thus, corresponding costs, and the trimming results in an additional scrap portion.
DE 30 38 865 C1 discloses a roll stand with a pair of work rolls which are axially displaceable in opposite directions and, if necessary, with intermediate rolls and back-up rolls, wherein, inter alia, the strip edge pressure can be reduced without effort. In this roll stand, each of the displaceable rolls has at least over a portion of the length of its roll body a curved contour which deviates from a straight line extending parallel to the axis, wherein the curve contour extends preferably over the entire length of the roll body and wherein the contours of the two rolls of the pair of rolls supplement each other without a gap exclusively in a certain axial position of the rolls. This makes it possible to influence the configuration of the roll gap and, thus the cross-sectional shape of the rolled strip even by carrying out small displacement paths of the rolls having the curved contours, and thus, to reduce the edge pressure for preventing the sharpening of the edges; however, the influence on the strip edges is possible only when a treatment of the entire strip is carried out.
In accordance with another known solution for reducing edge sharpening and the resulting scrap portion, a roll stand has been proposed with two work rolls which conically narrow at an end thereof, wherein one of these rolls is combined with another roll turned by 180.degree.. The pair of rolls is positioned in such a way that the strip edges of the rolling stock are located in the area of the beginning of the conical portion. Since the roll gap opens in the edge area as a result of the conical contour, the strip edge is being reduced to a lesser extent than would occur normally as a result of the roll flattening between the loaded part of the roll surface and the unloaded part next to the strip. However, this method has the disadvantage that the strip may crack when the strip slightly runs off as it is quite usual to happen. The reason for this is that the rolls are not moved up and, thus, a significantly decreased reduction and great tensile stresses occur on that side of the strip to which the strip runs.
Accordingly, for rolling mill operators the reduction of the scrap portion due to trimming resulting from the reduction of the edge drop is frequently not of major significance, but rather the increase of the operational safety, inter alia, by the avoidance of strip cracks. Experience has shown that these cracks can be prevented by long strip edges which may be slightly undulated and, thus, not subjected to tensile load. However, under usual conditions, an overproportional elongation of the strip limited to the strip edge area is not possible or only insufficiently possible by means of adjusting systems which influence the planar position, for example, roll displacement systems, roll bending systems or thermal systems. An exception are only rolls with very thin work rolls in which the planar position can be influenced near the strip edge; however, it remains essentially impossible to influence the strip center.
Therefore, the known measures are not sufficient to meet the increased requirements with respect to section accuracy and surface evenness of the strip edges and simultaneously to avoid strip cracks.